The Utilihab Designer software package is a modular 3D construction system that allows for the on-screen design and rendering of Utilihab structures using modular 3D elements available on-line and representing all the standard components and options offered by the Utilihab manufacturing community. It also accesses a public library of pre-developed structure designs created by end-users and professional designers for ready use and customization. Confined to the standard component grids, virtual components automatically snap into place on-screen according to their position and neighboring parts. The software prevents impossible combinations and also performs basic structural engineering analysis and automatically warns of impractical structural configurations. Relying for its component data on-line, the Utilihab Designer software is a web based application with an optional stand-alone front-end application and enjoys continuous update of its component and structure catalog through on-line publishing of manufacturer/designer component data.